Various microchips can be produced with a projection exposure apparatus and the formats of the masks on the reticle can be different. Accordingly, it is necessary to be able to variably and accurately delimit the illuminating spot on the reticle because, otherwise, exposure errors can occur. Mountings as close as possible to the reticle hinder the manipulation thereof and the requirements as to edge sharpness of the diaphragm action as it is needed for high-resolving structures cannot be satisfied. It is therefore known to provide an additional intermediate field plane in the illuminating beam path in which the reticle masking unit is precisely positioned. The additional lenses necessary therefor require a very substantial effort and additional space for equipment to meet the requirements with respect to image field, aperture, homogeneity and achromaticity.
European patent publication 0,297,161 discloses a projection exposure apparatus having a light-conducting glass rod in the illuminating beam path. A graded-density filter is mounted downstream of the glass rod and apparently provides for greater homogeneity but not for masking out light.